1. Field of the Invention
The invention relates to a burner, including a swirl generator for a combustion airflow, having a conical swirl chamber and means for admitting fuel into the combustion airflow, wherein the swirl generator incorporates combustion air inlet openings for the combustion airflow tangentially entering into the conical swirl chamber, and wherein the means for admitting fuel into the combustion airflow includes a first fuel feeding means having a first group of fuel outlet openings substantially disposed in the direction of the burner axis for a first premix fuel quantity.
2. Brief Description of the Related Art
A conical burner having a plurality of shells, a so-called double-cone burner, is known from EP 0 321 809 B1. The conical swirl generator, which is composed of a plurality of shells, generates a closed swirling flow in the cone head, which, due to the increasing swirl along the conically widening swirl chamber, becomes unstable and transitions into an annular swirling flow with a backflow in its core. The shells of the swirl generator are assembled in such a way that tangential air inlet slots for combustion air are formed along the burner axis. Along the inlet flow edge of the conical shells that is created in this manner, inlets for the pre-mix gas, i.e., the gaseous fuel, are provided, which incorporate outlet openings for the premix gas that are distributed in the direction of the burner axis.
The gas is injected through the outlet openings or bores at an angle to the air inlet slot. This injection process, in combination with the swirl of the combustion-air-fuel-gas generated in the swirl chamber, results in a good intermixing of the fuel gas or premix gas with the combustion air. In premix burners of this type a good intermixing is the precondition for low NOx values during the combustion process.
To further improve a burner of this type, a burner for a heat generator is known from EP 0 780 629 A2, which incorporates an additional mixing path adjoining the swirl generator, for an additional intermixing of fuel and combustion air. This mixing path may be implemented, for example, in the form of a downstream tube, into which the flow emerging from the swirl generator is transferred without any significant flow losses. With the aid of this additional mixing path the degree of intermixing can be increased further and the pollutant emissions reduced accordingly.
While it is true that the above burner systems, which operate based on the principle of the lean premix combustion, have low pollutant emissions, they additionally also have a noticeably limited stability range. Especially when low-caloric fuels, namely so-called MBTU and LBTU fuel gases are used, which flow through the fuel outlet openings along the combustion air inlet openings into the swirl chamber, it has been shown that the gas pre-pressure increases significantly, causing the degree of efficiency of a gas turbine system that is combined with a burner to be significantly reduced. Additionally, the flame speed increases significantly, bringing with it the risk of a flashback into the burner. In a constellation of this type the burner transitions into a so-called diffusion mode, which inevitably leads to high NOx emissions. In addition to the risk of a flashback into the mixing zone of the burner and lift-off and extinguishing of the premix flame, so-called thermo-acoustic oscillations result in a noticeably reduced burner performance.
One possible measure for countering the risk of a flashback provides for a fuel injection that is provided as far downstream as possible along the premixing path, causing the mixing path for the formation of a completely intermixed fuel-air mixture to be significantly shortened.
To improve the degree of intermixing between fuel and air, EP 0 918 191 A1 provides for a fuel injection that is offset back in the flow direction through the combustion air inlet openings, so that a partial intermixing occurs between the fuel and combustion air before they flow into the remaining interior space of the swirl generator. However, in order to ensure the formation of a stable flame front within the combustion chamber, extensive and structurally complicated flow-relevant measures must be taken within the burner structure. Additionally, it has been shown that especially the performance of a burner with respect to the flame stability, its emission values and the occurrence of thermo-acoustic pulsations are subject to much greater instabilities in burners for the operation of gas turbine systems with medium and small output than in the case of burners that are designed for high-efficiency gas turbine systems.